Polystyrenes are used in a variety of applications because they are excellent in rigidity, transparency, and gloss and have good moldability. However, these polystyrenes have a major disadvantage that they are inferior in impact resistance; various unvulcanized rubbers are used as tougheners in order to reduce the disadvantage. Among others, there is industrially widely produced a styrenic resin composition in which a styrenic monomer is graft polymerized to a rubber-like polymer, obtained by radical polymerization of the styrenic monomer in the presence of the unvulcanized rubber.
Among the unvulcanized rubbers used for this purpose are polybutadiene and styrene-butadiene copolymers; particularly, polybutadiene is widely used because it can confer excellent impact resistance.
In recent years, the styrenic resin composition has come to be required to have various excellent characteristics as it has applications spreading into housings and other parts for home electric appliances, axle parts, office machine parts, daily sundries, toys, and the like; a strong need exists for a styrenic resin composition excellent in the balance of appearance characteristics, rigidity, and impact resistance.
Generally, the styrenic resin composition is produced by dissolving, in a styrene monomer, polybutadiene rubber or styrene-butadiene copolymer rubber as a rubber-like polymer and performing a bulk polymerization or bulk-suspension polymerization method under stirring.
The improvement in impact resistance is generally made possible by increasing the content of the rubber-like polymer, but while a styrenic resin having the increased rubber-like polymer exhibits enhanced impact strength, it has reduced rigidity and gloss. On the other hand, the improvement in gloss is made possible by reducing the content of the rubber-like polymer or micronizing rubber-like polymer particles dispersed in the resin, but the impact resistance is markedly decreased.
Several methods for improving the impact strength of the styrenic resin composition have been previously disclosed. Examples of the disclosed methods include a method involving specifying the solution viscosity of the conjugated dienic polymer (see, for example, Japanese Patent Publication No. 58-04934), a method involving specifying the relationship between the solution viscosity and Mooney viscosity of the conjugated dienic polymer (see, for example, Japanese Patent Publication No. 53-44188), and a method involving specifying the relationship between the solution viscosity of the conjugated dienic polymer and the tensile modulus and swelling degree of an organic peroxide crosslinked product (see, for example, Japanese Patent Laid-Open No. 60-025001).
These methods improve the balance of impact resistance and gloss compared to conventional methods using polybutadiene, but the improvement has not always been satisfactory.
On the other hand, methods for improving the impact resistance and appearance characteristics using styrene-butadiene block copolymers with specific structures have been disclosed. (See, for example, Japanese Patent Laid-Open Nos. 61-143415, 63-165413, 02-132112, and 02-208312.) However, when examined in detail, these methods have not provided practically satisfactory balance of impact resistance and appearance characteristics.
There have been also disclosed a method for improving the impact strength of an impact-resistant styrenic resin using a conjugated dienic polymer modified with a nitrogen-containing compound comprising an oxygen atom in the molecule (see, for example, Japanese Patent Laid-Open No. 63-008411) and a method for improving the impact strength of an impact-resistant styrenic resin using a conjugated dienic polymer comprising a hydroxyl terminal group (see, for example, Japanese Patent Laid-Open No. 63-278920). In addition, there have been disclosed a method for improving the impact strength and gloss of an impact-resistant styrenic resin using a hydrogenated polymer modified with a hydroxyl, amino, epoxy, silanol or alkoxysilane group (see, for example, WO03/85010) and a method for improving the gloss and impact strength of a styrenic resin composition by combining a modified polymer and a silica-based inorganic filler (see, for example, Japanese Patent Laid-Open No. 2003-313255). However, when examined in detail, these methods have not provided the practically satisfactory balance of impact resistance and appearance characteristics. For example, the styrenic resin compositions obtained in the Examples of WO03/85010 and Japanese Patent Laid-Open No. 2003-313255 described above have not been satisfactory in appearance characteristics although they have improved impact resistance.
Thus, it has been difficult to sufficiently and well-balancedly improve the impact resistance, rigidity, and appearance characteristics of a conventional impact-resistant styrenic resin.